To make this type of dynamic relationship possible, the background needs to take into account the requirements of the foreground.  First, we finish the black and darker colors in the foreground shape and finish making all decisions about its shading.  The background is then applied using mostly medium and light tones, as not to compete with the deeper foreground shapes.  This color is pulled up against the outline of the highlighted upper parts of the foreground shape, establishing a strong neg-on-pos relationship, with the darkest tones closest to the outline.  In other parts of the background, the color is applied lighter as it gets closer to the shaded underparts of the piece, leaving them a strong pos-on-neg, usually with some blank skin behind the darkest part of each foreground shape.  Throughout this process, the background is kept smooth and simple in such a way that doesn’t compete with the foreground.

Fig. 33a

In Fig. 33a is the same snail that we saw earlier, but this time we’re using a dynamic pos/neg relationship. The upper parts of the snail, which are lit by the sun above, look bright because of the deep tones right behind them.  The undersides of each foreground shape are then brought to a deep shadow against the edge; wherever this happens, the background is made to drop out, allowing the deep tones in the snail to stand bold and clear.  The light and dark areas of the background are made to merge smoothly together, drawing attention away from its fluctuations in value, keeping the snail focused and in the foreground.

In Fig. 33b is a piece where we opted for a dynamic pos/neg relationship, where the undersides of forms in the foreground were shaded dark, creating a pos- on-neg relationship, while their upper sides were highlit in light colors and a medium background pulled behind them, giving these upper parts a clear neg-on-pos readability.  Without the background, these highlit areas would have been neg-on-neg, making the piece look unfinished.

Fig. 33b
 

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